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Rollup merge of #108203 - compiler-errors:rpitit-fix-defaults-2, r=jackh726

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Fix RPITITs in default trait methods (by assuming projection predicates in param-env)

Instead of having special projection logic that allows us to turn `ProjectionTy(RPITIT, [Self#0, ...])` into `OpaqueTy(RPITIT, [Self#0, ...])`, we can instead augment the param-env of default trait method bodies to assume these as projection predicates. This should allow us to only project where we're allowed to!

In order to make this work without introducing a bunch of cycle errors, we additionally tweak the `OpaqueTypeExpander` used by `ParamEnv::with_reveal_all_normalized` to not normalize the right-hand side of projection predicates. This should be fine, because if we use the projection predicate to normalize some other projection type, we'll continue to normalize the opaque that it gets projected to.

This also makes it possible to support default trait methods with RPITITs in an associated-type based RPITIT lowering strategy without too much extra effort.

Fixes #107002
Alternative to #108142
This commit is contained in:
Guillaume Gomez 2023-02-19 14:47:56 +01:00 • committed by GitHub
commit d2aef58eb5
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6 changed files with 161 additions and 37 deletions
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36
compiler/rustc_trait_selection/src/traits/project.rs
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@ -90,15 +90,7 @@ enum ProjectionCandidate<'tcx> {
/// From an "impl" (or a "pseudo-impl" returned by select)
Select(Selection<'tcx>),
ImplTraitInTrait(ImplTraitInTraitCandidate<'tcx>),
}
#[derive(PartialEq, Eq, Debug)]
enum ImplTraitInTraitCandidate<'tcx> {
// The `impl Trait` from a trait function's default body
Trait,
// A concrete type provided from a trait's `impl Trait` from an impl
Impl(ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>>),
ImplTraitInTrait(ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>>),
}
enum ProjectionCandidateSet<'tcx> {
@ -1292,17 +1284,6 @@ fn assemble_candidate_for_impl_trait_in_trait<'cx, 'tcx>(
let tcx = selcx.tcx();
if tcx.def_kind(obligation.predicate.def_id) == DefKind::ImplTraitPlaceholder {
let trait_fn_def_id = tcx.impl_trait_in_trait_parent(obligation.predicate.def_id);
// If we are trying to project an RPITIT with trait's default `Self` parameter,
// then we must be within a default trait body.
if obligation.predicate.self_ty()
== ty::InternalSubsts::identity_for_item(tcx, obligation.predicate.def_id).type_at(0)
&& tcx.associated_item(trait_fn_def_id).defaultness(tcx).has_value()
{
candidate_set.push_candidate(ProjectionCandidate::ImplTraitInTrait(
ImplTraitInTraitCandidate::Trait,
));
return;
}
let trait_def_id = tcx.parent(trait_fn_def_id);
let trait_substs =
@ -1313,9 +1294,7 @@ fn assemble_candidate_for_impl_trait_in_trait<'cx, 'tcx>(
let _ = selcx.infcx.commit_if_ok(|_| {
match selcx.select(&obligation.with(tcx, trait_predicate)) {
Ok(Some(super::ImplSource::UserDefined(data))) => {
candidate_set.push_candidate(ProjectionCandidate::ImplTraitInTrait(
ImplTraitInTraitCandidate::Impl(data),
));
candidate_set.push_candidate(ProjectionCandidate::ImplTraitInTrait(data));
Ok(())
}
Ok(None) => {
@ -1777,18 +1756,9 @@ fn confirm_candidate<'cx, 'tcx>(
ProjectionCandidate::Select(impl_source) => {
confirm_select_candidate(selcx, obligation, impl_source)
}
ProjectionCandidate::ImplTraitInTrait(ImplTraitInTraitCandidate::Impl(data)) => {
ProjectionCandidate::ImplTraitInTrait(data) => {
confirm_impl_trait_in_trait_candidate(selcx, obligation, data)
}
// If we're projecting an RPITIT for a default trait body, that's just
// the same def-id, but as an opaque type (with regular RPIT semantics).
ProjectionCandidate::ImplTraitInTrait(ImplTraitInTraitCandidate::Trait) => Progress {
term: selcx
.tcx()
.mk_opaque(obligation.predicate.def_id, obligation.predicate.substs)
.into(),
obligations: vec![],
},
};
// When checking for cycle during evaluation, we compare predicates with